This would seem to indicate that the insulator blocks have been in use under the carburetors since the very first model years. It is recommended that a carburetor gasket also be installed above and below the insulator to ensure a proper seal and to avoid vacuum leaks. The insulators themselves can develop cracks over time, and can also be damaged when they are removed from the intake manifold. The insulator should be carefully inspected to ensure that no small cracks have formed that could produce a vacuum leak at the base of the carburetor.

The issue with heat buildup in the Rochester carburetor is also mentioned on page 6 of the attached Delco Rochester Carburetor Service Manual, as follows…

MAIN WELL INSERTS

During very hot operation, the fuel in the main fuel well tends to boil and this produces vapor bubbles. The bubbles tend to interfere with fuel flowing into the main well tube which upsets good metering. To help reduce the effect of heat and remove the fuel vapor bubbles, an aluminum main well insert is used. This resembles the main well tube and completely surrounds it. This reduces the amount of vapor getting into the main well tube, helps dissipate heat and improves hot operation of the system.

So it would seem that heat transfer from the air-cooled Corvair heads via the integrated aluminum intake manifolds to the carburetor mounting pads and on to the carburetor bodies was a well-known problem to Chevrolet engineers, and as such it was addressed in several ways — using insulator blocks between the intake manifold and the carburetors, and internally in the design of the Rochester H carburetors using an aluminum main well insert.